Mobile offshore drilling unit

ABSTRACT

A semisubmersible mobile offshore drilling unit (MODU) includes a submersible lower hull comprising a ring pontoon having a trapezoidal shape; an upper hull having the trapezoidal shape; three stability columns supporting the upper hull from the lower hull; and a drilling rig. The MODU may be operated in a tender assist mode or an independent drilling mode. The MODU may be equipped to prepare, drill, and complete a subsea wellbore or to workover an existing subsea wellbore.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to a mobileoffshore drilling unit.

2. Description of the Related Art

FIG. 1A illustrates a drilling rig 6. A drilling rig 6 is a portablefactory for forming deep wellbores 40 in the ground. When a drill bit 10is pressed against the ground and rotated, the teeth on the bit 10 grindand gouge the rock into small pieces. These pieces of rock or cuttingsmust be moved out of the way so the bit teeth can be constantly exposedto fresh, uncut rock. Drilling fluid, such as mud, is used to move thecuttings away from the bit 10. A mud pump 20 takes mud from mud tanks 22and pumps it under high pressure up a standpipe 24, through the swivel26, down the kelly 54, down the drill pipe 30, through the drill collars32 and out jets in the bit 10. Mud, exiting under pressure from jets inthe bit 10, clears the cuttings and moves then up the annulus 42 of thewellbore 40. The mud and cuttings are then passed over a shale shaker 44which separates the cuttings from the mud and allows the mud to returnto the mud tank 22 for recirculation. The cuttings are sampledperiodically for geologic purposes, but most are discarded.

Since the drill bit 10 is usually rotated to form the wellbore, drillingrig 6 usually includes a rotary system. The rotary system includesdrilling floor 50 and rotary table 52. Located near the center of therotary table 52 is a kelly bushing and kelly 54. In many modern drillingunits, a top drive motor (not shown) positioned in the derrick on railsreplaces the rotary table, kelly bushing, and kelly for impartingrotation to the drill string. Rotational force is transferred from therotary system to the drill string including drill pipe 30 and drillcollars 32. The drill collars 32 are heavy and stiff and assist inmaintaining the bit in a vertical position. The weight of the drillcollars 32 applied directly above the bit assists in increasing thecutting ability of the bit 10.

FIG. 1B illustrates the draw works 68 of the drilling rig 6. Wells arenow being drilled to depths in excess of 30,000 feet. The tremendousweight of the drill string must be supported by a substantial derrick60. A drilling line 64 is attached to traveling block 66, sheaved overcrown block 62 and attached to draw works 68. The draw works 68 includesa winch which is used to hoist the drill string, including the drillpipe 30, drill collars 32 and bit 10, out of the hole. Any drilling unitneeds power to the turn the bit, power to drive the mud pump and powerto run all the ancillary machinery. The power system on most offshorerigs is usually diesel/electric. The prime movers, being diesel, areused to drive generators to generate electric power which is used topower the other equipment.

Mobile offshore drilling units (MODUs) move from one drill site on thewater to another. There are two basic types of MODUs used to drill mostoffshore wells: (1) bottom supported units including submersibles andjack-ups; and (2) floating units including lake barge rigs, drill ships,and semi-submersibles.

FIG. 1C illustrates a semi-submersible MODU 70. Floating units aretypically used in water depths greater than where a bottom supportedunit is capable. A semi-submersible 70 has a drilling rig 71 mounted inthe middle and includes an opening, referred to in the industry as amoon pool (not shown), through which drilling operations are conducted.Semi-submersibles 70 include a lower barge hull 74 which floats belowthe surface of the sea 72 and is, therefore, not subject to surface waveaction. Large stability columns 73 mounted on the lower barge hull 74support the upper hull 78, which includes a main deck and machinery deckabove the surface of the water 72.

SUMMARY OF THE INVENTION

Embodiments of the present invention generally relate to a mobileoffshore drilling unit. In one embodiment, a semisubmersible mobileoffshore drilling unit (MODU) includes a submersible lower hullcomprising a ring pontoon having a trapezoidal shape; an upper hullhaving the trapezoidal shape; three stability columns supporting theupper hull from the lower hull; and a drilling rig.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1A illustrates a drilling rig. FIG. 1B illustrates the draw worksof the drilling rig. FIG. 1C illustrates a semi-submersible mobileoffshore drilling unit (MODU).

FIG. 2A is a perspective view of a semi-submersible MODU in a drillingmode, according to one embodiment of the present invention. FIG. 2B is aplan view of the semi-submersible MODU in drilling mode. FIG. 2C is aprofile view of the semi-submersible MODU in drilling mode.

FIGS. 3A-G illustrate a drilling rig, a.k.a. derrick equipment set(DES), of the MODU. FIG. 3A is a profile view of the DES. FIG. 3B isanother profile view of the DES. FIG. 3C is a profile view of the drillpipe handling system. FIG. 3D is a plan view of the drill floor. FIG. 3Eis a plan view of the process level. FIG. 3F is a plan view of the lowerlevel. FIG. 3G is a plan view of the base.

FIG. 4A is a plan view of a mezzanine deck of the MODU. FIG. 4B is aplan view of a machinery deck of the MODU. FIG. 4C is a plan view of atank deck of the MODU. FIG. 4D is a plan view of a lower level of thecabin. FIG. 4E is a plan view of an upper level of the cabin. FIG. 4F isa plan view of the helipad.

FIGS. 5A-E illustrate the DES being deployed in tender assist drillingmode. FIG. 5A is a plan view of the main deck. FIG. 5B is a plan view ofthe MODU connected to a platform. FIG. 5C is a profile view of the DEShaving been hoisted from the MODU and set on the platform. FIG. 5D is aprofile view of the DES in place over the platform's moonpool. FIG. 5Eis a perspective view of the MODU in tender assist drilling mode.

DETAILED DESCRIPTION

FIG. 2A is a perspective view of a semi-submersible MODU 100 in adrilling mode, according to one embodiment of the present invention.FIG. 2B is a plan view of the semi-submersible MODU 100 in drillingmode. FIG. 2C is a profile view of the semi-submersible MODU 100 indrilling mode.

The MODU 100 may be semi-submersible and include a helipad 101, an upperhull 105, one or more stability columns 110, a submersible lower hull115, one or more cranes 120 a-c, one or more life-rafts 125, a cabin130, a remotely operated vehicle (ROV) 135, a mooring system 140, adrilling rig, a.k.a. derrick equipment set (DES), 150, and a moonpool155. The helipad 101 may be used for shuttling roughnecks to and fromthe MODU 100. The upper hull 105 may have an trapezoidal shape, such asisosceles trapezoidal, and may include one or more decks, such as a maindeck 105 p, a mezzanine deck 105 z, a machinery deck 105 m, and a tankdeck 105 t. As discussed above, the upper hull 105 and stability columns110 may float on the lower hull 115 and three stability columns maysupport the upper hull from the lower hull. The lower hull 115 mayinclude one or more pontoons and a ballast system. The pontoon may be aring pontoon having a trapezoidal shape corresponding to the upper hull.The pontoons may float near or on the surface when empty for towing theMODU 100 between locations and may submerge when ballasted for drillingor tender assist drilling mode (discussed below). The ROV 135 may bedeployed to perform subsea work. The mooring system 140 may be employedto maintain position of the MODU 100 during drilling or tender assisteddrilling. Alternatively or additionally, the MODU 100 may include adynamic positioning system to maintain position of the MODU 100.

FIGS. 3A-G illustrate the DES 150 in drilling mode. FIG. 3A is a profileview of the DES 150. The DES 150 may include a derrick 205, a top drive210, a drill floor 215, a process level 220, a lower level 225, a base230, drawworks 235, a crown block 240, a blow out preventer (BOP) 245(see FIG. 3B), a drill pipe handling system 250, a mud treatment system255, a driller's cabin 265 (see FIG. 3D), and a skidding system 275. Thederrick 205, drill floor 215, process level 220, and lower level 225 maybe mounted on the base 230, such as by fasteners. In the drilling mode,the derrick 205 may be located over the moonpool 155 to conduct drillingoperations and the base 230 may rest on the main deck 105 p and/or befastened to the upper hull 105.

FIG. 3B is another profile view of the DES 150. The derrick 205 mayinclude an upper mast portion 205 m and lower housing portion 205 h. Indrilling mode, the mast 205 m may extend from the housing 205 h. Theportions 205 h,m may be connected, such as fastened, at an overlap 205c. To transport the derrick 205 for tender assist drilling mode or whentowing the MODU 100 between locations, one of the cranes 120 a-c may befastened to the mast 205 m, the connection 205 c disassembled, and themast 205 m may then be lowered into the housing. The connection 205 cmay be reassembled, the derrick 205 may be disconnected from the base230, and the derrick 205 may be laid down on the main deck 105 p (seeFIG. 5A).

FIG. 3C is a profile view of the drill pipe handling system 250. FIG. 3Dis a plan view of the drill floor 215. The drill pipe handling system250 may include a bridge crane, a racking arm, a racking board, anelevator, one or more power tongs (IR), an offline stand building (OSB),and one or more pipe bins. Single joints of drill pipe 260 may be storedin the pipe bins. During and before drilling, the joints 260 may beassembled into stands of two or more joints, such as quad stands. Thejoints 260 may be assembled by retrieving a joint from the bins using acatwalk machine, raising the joint to a vertical position using theelevator, setting the joint into a mouse hole, and engaging slips of themouse hole spider. A second joint may then be hoisted over the firstjoint in a similar fashion. The power tongs may engage the joints andmake-up a threaded connection between the two joints. This process maybe repeated until a quad stand is assembled. The racking arm may thenretrieve the assembled stand and the bridge crane may lift the rackingarm to store the stand in the racking board.

Once the drill string has drilled to an end of a capable depth, the topdrive may raise the drill string and set the drill string in a spider ofa rotary table in the drill floor. The top drive may then bedisconnected from the drill string and raised to a height proximate tothe racking arm. The racking arm may retrieve the stand from the rackingboard and the bridge crane may lift the racking arm and the stand andhold the stand proximate to the top drive. The top drive may then engagethe stand with a backup tong, and connect the stand to the quill. Thetop drive may then lower the stand to an end of the drill string. Asecond set of power tongs may engage the stand and make-up a threadedjoint between the stand and the drill string. While the top drive isdrilling with the extended drill string, the handling system 250 may beassembling and storing more stands as needed.

The drill floor 215 may further include other accessories, such as ahydraulic power unit, an air compressor, a mud logger building, ameasurement while drilling (MWD) building, a choke manifold, and astandpipe manifold.

FIG. 3E is a plan view of the process level 220. FIG. 3F is a plan viewof the lower level 225. The mud treatment system 255 may include aprocess tank, one or more shale shakers, a degasser, a desander, adesilter, a centrifuge, one or more feed pumps, an agitator, one or moretrip tanks, and a gas separator. The process tank may include a sandcompartment, a degas compartment, a desand compartment, a desiltcompartment, and a clean mud compartment.

FIG. 3G is a plan view of the base 230. The base 230 may include a frameoperably coupled to the skidding system 275. The skidding system mayinclude one or more linear actuators and one or more skid beams. Eachlinear actuator may be connected to the base and a respective skid beam.Each linear actuator may be hydraulic, such as a piston and cylinderassembly, or electric, such as a motor and ball-screw assembly. The skidbeams may rest on the main deck 105 p in drilling mode. The base 230 maybe slidable on the skid beams by operation of the linear actuators.

Additionally, the MODU 100 may include riser pipe for assembly anddeployment of a riser string (see FIG. 2C). Further, the MODU mayinclude production tubing (not shown) for assembly and deployment of aproduction tubing string. Further, the MODU may include a subseawellhead and a subsea production tree (a.k.a. Christmas tree), such as avertical or horizontal tree for deployment to the wellhead. The MODU 100may prepare, drill, and complete a subsea wellbore. Alternatively, theMODU 100 may be deployed for workover of an existing subsea wellbore.

FIG. 4A is a plan view of the mezzanine deck 105 z. The mezzanine deck105 z may include crew quarters and crew facilities, such as a washroom,toilets, locker room, and utility room.

FIG. 4B is a plan view of the machinery deck 105 m. The machinery deck105 m may include a diesel/electric power system including dieseldrivers and electric generators and a plurality of electrical controlpanels. The machinery deck may further include one or more mud pumps andone or more mud tanks, one or more reserve mud tanks, pre-load/ballasttanks, dry mud containers, dry cement containers, one or more cementblenders, and one or more cement pumps. The machinery deck may furtherinclude additional crew quarters and facilities, such as a galley, messhall, cinema, and gym.

FIG. 4C is a plan view of the tank deck 105 t. The tank deck 105 t mayinclude mud pit cellar tanks, drilling water tanks and fuel oil tanks.The tank deck may further include additional pre-load/ballast tanks.Further, electrical cables and piping (not shown) may extend between thedecks 105 p,z,m,t and the DES 150 for transferring fluids andelectricity.

FIG. 4D is a plan view of a lower level 130 l of the cabin 130. FIG. 4Eis a plan view of an upper level 130 u of the cabin 130. FIG. 4F is aplan view of the helipad. The cabin 130 may include crew quarters, ahospital, offices, a lounge, an emergency generator, a utility room, acontrol house, a conference room, a waiting area for the helipad, and anHVAC system.

FIGS. 5A-D illustrate the DES 150 being deployed in tender assistdrilling mode. FIG. 5A is a plan view of the main deck 105 p. In tenderassist drilling mode, the DES 150 may be located on an adjacentproduction platform 500. The MODU 100 may be connected to the platform500 as if the DES were in drilling mode, supplying drilling fluid andelectricity to the DES 150. The MODU 100 may be connected using one ormore hawsers and one or more umbilicals. Tender assisted drilling may beuseful for workover of existing wells having the existing productionplatform 500 in place and connected to a subsea production tree (notshown). Otherwise, the existing production platform 500 would have to bedisconnected from the subsea production tree and moved so that the MODU100 could be moved over the production tree and connected to the tree.To move the DES 150 from the MODU 100 to the platform 500, the DES 150may be disassembled as discussed above. The DES 150 may be disassembledprior to towing the MODU 100 to the platform 500. The MODU 100 mayfurther include a telescopic personnel bridge 505 rotatable about themain deck 105 p for deployment to the platform 500.

FIG. 5B is a plan view of the MODU 100 connected to a platform 500. FIG.5C is a profile view of the DES 150 having been hoisted from the MODU100 and set on the platform 500. FIG. 5D is a profile view of the DES150 in place over the platform's moonpool. FIG. 5E is a perspective viewof the MODU 100 in tender assist drilling mode. The MODU 100 may bemoored and/or dynamically positioned in place adjacent the platform 500.The telescopic bridge 505, hawsers, and umbilicals may be deployed tothe platform 500. The base 230, drilling floor 215, process level 220,and lower level 225 may then be hoisted by the crane 120 a onto theplatform 500. The crane 120 a may include an elevator and a tower sothat the crane boom may be raised on the tower to accommodate heightdifference between the MODU 100 and the platform 500. The derrick 205may then be hoisted by the crane 120 a onto the platform 500. The DES150 may then be assembled. The DES 150 may be assembled on an edge ofthe platform 500 and skidded to the platform's moonpool using theskidding system 275. The drill pipe 260 may be hoisted from the MODU tothe platform and loaded into the rig's bins.

Alternatively, instead of the drilling rig, the MODU 100 may include aJ-lay tower (not shown) pivotably mounted to the main deck over themoonpool. The J-lay tower may be transported in a horizontal positionand then raised to a vertical or near-vertical position for deploymentof a subsea pipeline. Alternatively, the MODU 100 with the J-lay towermay be used to lay tendons for a tension leg platform.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A semisubmersible mobile offshore drilling unit (MODU), comprising: asubmersible lower hull comprising a ring pontoon having a trapezoidalshape; an upper hull having the trapezoidal shape; three stabilitycolumns supporting the upper hull from the lower hull; and a drillingrig.
 2. The MODU of claim 1, further comprising a crane operable tohoist the drilling rig to a platform for a tender assist mode; and anumbilical for powering the drilling rig from the MODU in the tenderassist mode.
 3. The MODU of claim 1, further comprising a moonpool,wherein the drilling rig is deployable over the moonpool for a drillingmode.
 4. The MODU of claim 1, wherein the MODU is operable to prepare,drill, and complete a subsea wellbore.
 5. The MODU of claim 1, whereinthe MODU is operable to workover a subsea wellbore.